1. Field of the Invention
The present invention relates to a water-lubricated bearing apparatus for hydraulic machinery.
2. Description of the Prior Art
Hydraulically operated turbines, in particular such large-scaled turbines as hydraulic turbines and reversible pump-turbines, are in general provided with a group of guide vanes disposed around a runner, so that water flowing into the turbine is guided to the runner with its quantity being controlled, so as to rotate and drive the runner. The rotation of the runner is transmitted to the generator via a main shaft to thereby generate electricity. The main shaft is provided with a guide bearing in order to prevent any vibration of the runner, the generator and the main shaft during their operation and thus to attain their stable rotation.
The structure of a conventional guide bearing will be explained with reference to FIG. 1 illustrating the structure of an oil-lubricated bearing which is a typical known guide bearing and is used in, for example, a Francis turbine.
A guide vane 1 is disposed between an upper cover 2 and a lower cover 3. A runner 4 is disposed within the space defined by the covers 2 and 3 at a location downstream of the guide vane 1. The runner 4 comprises a runner crown 5 closer to the upper cover 2, a runner blade 6, and a runner band 8 closer to the lower cover 3.
The runner 4 is mounted on a main shaft 9 extending upward from the runner 4, in the following manner. The runner crown 5 is brought into contact with a flange portion 10 formed on the main shaft 9 and a bolt 11 is threaded, so as to connect the runner crown 5 and the flange portion 10 together, thereby mounting the runner 4 on the tip portion of the main shaft 9. A water sealer 12 is provided, which extends from the upper cover 2 toward the bolt 11 and surrounds the main shaft 9 with a gap therebetween. A part of the main shaft 9 which is above the water sealer 12 is formed with a main shaft skirt 16 at a position partially determining an overhung dimension H (described later). The main shaft skirt 16 is enclosed by an oil tank 17.
One edge portion of the oil tank 17 is disposed in a groove 16A formed in the main shaft skirt 16 on the side thereof closer to the water sealer 12, while the other edge portion of the oil tank 17 is disposed in correspondence with a part of the main shaft 9 which is above the main shaft skirt 16. These portions of the oil tank 17 are integrated by an intermediate body surrounding the main shaft skirt 16, thereby forming the oil tank 17. A flange portion 17A is formed on the outer periphery of the oil tank 17. The flange portion 17A is brought into contact with a flange portion 2C of a supporting portion 2B extending upward from the upper cover 2, and they are fastened by a fastener (not shown), whereby the flange portion 17A of the oil tank 17 is supported by the flange portion 2C of the upper cover 2. The inside of the oil tank 17 is filled with lubricating oil 18, and a bearing apparatus 20 is disposed within the oil tank 17.
The bearing apparatus 20 has a supporting portion 20A mounted on the inner periphery of the oil tank 17 at a position corresponding to the flange portion 17A. The supporting portion 20A opposes the main shaft skirt 16 nd is provided with adjusting bolts 21 (only one of which is shown). The adjusting bolts 21 press against and thus support a segment bearing 22 which in turn presses against the main shaft skirt 16.
When the runner 4 and the main shaft 9 rotate, the main shaft skirt 16 also rotates as it slides on the segment bearing 22. Frictional heat generated during this sliding rotation is cooled by the lubricating oil 18. Part of the oil heated is cooled by cooler tubes 23.
The overhung dimension H is a distance which is inevitably necessary for the purposes of, for instance, enabling the maintenance of the sealer 12 disposed between the bearing apparatus 20 and the runner 4, ensuring a sufficient space for assembling and disassembling, and coping with various dimensional limitations on the mounting of the cooler tubes 23 within the oil tank, which tubes have to be provided for cooling heat generated from the bearing apparatus 20.
On the other hand, from the viewpoint of the vibration characteristics of the main shaft 9, the overhung dimension H should be as small as possible. Further, the use of oil as the lubricant necessitates the periodic checking and replacement of oil, thus being disadvantageous in terms of maintenance. The use of oil also necessitates the provision of the cooler tubes and makes the overall structure complicated.
In order to overcome these problems, a proposal has previously been made by Japanese Pat. Laid-Open No. 51-22954. According to this proposal, a bearing apparatus is a water-lubricated bearing apparatus to which water is supplied. This apparatus, however, encounters some problems. For instance, since an external clear water source and a device for supplying water therefrom are necessary, the bearing apparatus has a complicated structure. Further, if a natural water source is used, there is a risk of the water-lubricated surfaces being worn because water flowing into the runner at the time of a flood contains earth and sand. In order to avoid such a risk, Japanese Pat. Laid-Open No. 60-88215 discloses a bearing which is operable in water containing earth and sand without requiring any supply of clear water from the outside. This bearing is formed of new ceramic materials, which have been highlighted in recent years, and bearings of this type are used in pumps which deal with water containing earth and sand, such as drainage pumps.
The proposed bearing has an integrated journal bearing. Materials which may be used to form the journal bearing are silicon nitride, silicon carbide, etc. which, among various new ceramic materials, generate a relatively small underwater friction force and experience a relatively little wear when exposed to earth and sand. Materials which may be used to form the main shaft inserted through the journal bearing are very hard alloys, such as tungsten carbide, from the viewpoint of the resistance to earth-and-sand wear and the ease of mounting.
If a main shaft is rotated in a water containing earth and sand while the shaft is supported by a bearing, the main shaft becomes worn by earth and sand and thus has its life curtailed. With a view to lengthening the life of a main shaft, the present inventors have conducted experiments, applying a ceramic material to the main shaft. As a result, the present inventors have found that, if a ceramic material is simply used, it becomes broken, proving such application of a ceramic material to be impossible.